Recently, from the viewpoint of simple and inexpensive image production, inkjet recording has been used in a variety of printing fields including photographs, prints, marking, and special prints like color filters. Examples of inkjet inks used for inkjet recoding include aqueous inkjet ink containing water as a main solvent. Further, there are oil-based inkjet inks substantially free from water, which contain mainly a non-volatile solvent which does not volatilize at room temperature; non-aqueous inkjet inks substantially free from water, which contain mainly a solvent which volatilizes at room temperature; and hot-melt inks which are solid at room temperature and melt by heating for printing. Actinic radiation curable inkjet inks are also known that are cured by irradiation with actinic rays such as UV rays after printing and are put into practical use in applications including coating compositions for plastics, paper, wood and inorganic materials, adhesives, printing inks, printed circuit boards, electrical insulation, and other related applications
As a method for increasing the pinning property of an UV curable inkjet ink, for example, addition of gelling agents for temperature-induced sol-gel phase transition has been studied. Specifically, studies have been made to prevent combining of dots by discharging liquid ink droplets at high temperature and cooling the ink droplets for gelation at the same time they are landed on a recording medium. As for the gelling agent to be added to an ink, stearone and the like are disclosed (see PTL 1 and PTL 2).
Examples of light sources for photocuring of the UV curable inkjet ink landed on a recording medium include a high pressure mercury lamp, an LED, and a low pressure mercury lamp. UV curable inks which can be cured using an LED or a low pressure mercury lamp, which features low power consumption, have been desired. However, when a light source with low illuminance such as LED is used for an UV curable inkjet ink, a cured ink film may easily have insufficient degree of curing. For such reasons, the cured film becomes sticky. Although the stickiness of the cured film of an UV curable inkjet ink can be lowered by adding a gelling agent, the strength of the cured film is insufficient.
Addition of an amine-modified oligomer or branched polymer to an inkjet ink for improved curability is described (refer to PTL 3 and PTL 4).